The present invention relates to an apparatus and method for mixing material including solids and gaseous materials in an open surface body of liquid.
Floating vertical shaft downflow mixers such as disclosed in U.S. Pat. Nos. 4,422,771 and 3,856,272 and U.K. Pat. No. 1,428,349 have heretofore been provided for mixing materials in an open surface body of liquid. In general, such prior floating vertical shaft downflow mixers were arranged to pump liquid from an upper inlet downwardly to an annular discharge outlet which discharged the liquid in an annular generally conical stream radially symmetrical with respect to the axis of the propeller drive shaft. Such downflow mixers produce a circulation of liquid in an annular area around the mixer, but are limited in circulating and mixing in areas remote from the mixer. It has also been proposed as disclosed in U.S. Pat. No. 2,991,983, to provide a fixedly mounted vertical shaft directional mixer in which the propeller is arranged to pump liquid downwardly through a draft tube to a lateral discharge outlet at the lower end of the draft tube and discharge the liquid in a unidirectional stream extending in a relatively narrow arc from the mixer. Concentration of the energy of the liquid discharged from the mixer into a relatively narrow arc markedly increases the distance that the stream will travel away from the mixer and improves circulation and mixing over a larger area of the basin. In the fixedly mounted mixer disclosed in that patent, the direction of the unidirectional stream can be changed by rotating the draft tube about an upright axis.
It is more convenient and practical in many installations to provide a float supported mixer. Such floating mixers are easier to install and also accommodate changes in the water level in the basin. However problems were encountered in stabilizing a floating vertical shaft directional downflow mixer. In floating vertical shaft downflow mixers, the motor is mounted on the float to extend upwardly from the float and because of the very substantial weight of the motor as compared to the other parts of the mixer, the center of gravity of floating vertical shaft downflow mixer is above the center of buoyancy of the mixer. When a directional draft tube was added to the floating vertical shaft downflow mixer, the reaction forces produced by the laterally directed stream of liquid discharged from the lateral outlet of the draft tube tended to cause the floating mixer to tilt excessively. Attempts to overcome the stability problem by providing a plurality of floats arranged so as to increase the buoyancy of the float at the side at which the draft tube discharged the water, were marginally successful but required a complicated float structure. In addition, increasing the buoyancy of the float at one side of the mixer caused the mixer to float with the propeller shaft at an angle to the vertical when the mixer was not in operation, but was also not entirely successful in stabilizing the mixer when the mixer was in operation.